1. Field of the Invention
This invention relates to a superhigh-speed, superhigh-frequency biopolar transistor device.
2. Description of the Prior Art
The current gain cut-off frequency f.sub.T and the power gain cut-off frequency f.sub.m of a bipolar transistor (BT) are expressed as follows. ##EQU1## where .tau.e (emitter depletion layer charging time)=re (Cbc+Ceb), .tau.b (base transit time of minority carriers)=Wb.sup.2 /2Db, .tau.c (collector depletion layer transit time of minority carriers)=Wc/2Vs, .tau.cc (collector depletion layer charging time)=(Ree+Rc) Cbc, Rb: base resistance, Cbc: base-collector capacitance, Ceb: base-emitter capacitance, Wb: base layer thickness, Db: base diffusion coefficient of minority carriers, Wc: collector depletion layer thickness, Vs: collector saturation velocity of minority carriers, re: emitter resistance, Ree: emitter-contact resistance, and Rc: collector resistance. &lt;Large values of f.sub.T and f.sub.m are required for high speed operation of BT.&gt;
In the BT, as is clear from the formulae above, to increase f.sub.T and f.sub.m, it is necessary to decrease the capacitances of Cbc and Ceb, the thickness of base layer, the base resistance, the emitter resistance, and the collector resistance. In particular, to obtain a large f.sub.m, it is necessary to reduce Rb and Cbc. For these purposes, it is extremely important to reduce the size of each part of BT, optimize the electrode layout, and optimize the process to lower the contact resistance of the electrodes, and various attempts have been made in these directions.
It has been reported by H. Kroemer that the heterojunction bipolar transistor (HBT) using a semiconductor material having a wider energy band gap than that of the base as the emitter is intrinsically superior to an ordinary BT (H. Kroemer, "Heterostructure Bipolar Transistors and Integrated Circuits", Proc. IEEE, vol. 70, P. 13, 1982). In the HBT, injection of holes from the base to the emitter is restricted (in the case of an NPN transistor), and hence it is possible to construct a base having a higher doping density and having an emitter and collector at a lower doping density than those of an ordinary BT. Therefore, it is intrinsically advantageous for the reduction of the base resistance Rb, the emitter-base junction capacitance Cjeb and the base-collector junction capacitance Cjbc. Since the emitter and collector are at low doping densities and the base is at a very high doping density, Cjeb and Cjbc are expressed as follows: ##EQU2## where ne and nc are the doping density concentrations of the emitter and collector, and Ajeb and Ajbc are the junction areas of emitter and base, and base and collector, respectively. Therefore, Cjeb and Cjbc can be intrinsically reduced.
However, to utilize the above-described features effectively, the device structure and process should be optimized to reduce parasitic elements of resistance and capacitance. For this purpose, some methods have been attempted.
P. M. Asbeck et al. has reported a method to reduce the collector size by implanting oxygen ions into the collector layer. (P. M. Asbeck et al., "GaAs/(Ga, Al)As Heterojunction Bipolar Transistors with Buried Oxygen-Implanted Isolation Layer", Election Device Lett., vol. EDL-5, P. 310, 1984). However, the following problems occur in this method. A smaller mask than the buried collector is additionally required to make an emitter electrode. In this case, when the transistor size is made smaller, the mask alignment between the buried collector and the emitter electrode is more difficult. Furthermore, for a metal delineation from the emitter electrode, a different mask may be necessary. This may cause another problem in mask alignment between the emitter electrode and the metal delineation or contact lead when the transistor size is reduced, and may cause a step structure of the delineation metal at an end of the emitter electrode, and hence may cause the breakage of the delineation metal there.
Nagata et al. has reported a method to form the base electrode very close to the emitter portion by self-alignment and to reduce the external base resistance (Nagata et al., "A New Self-aligned Structure AlGaAs/GaAs HBT for High Speed Digital Circuits," Proc. Symp. on GaAs and Related Compounds (Inst. Phys. Conf. Ser. 790, P. 589, 1985). However, in this process and the structure by it, an emitter mesa and a base mesa are formed, and an emitter electrode is on the emitter mesa and a base electrode is formed very close to the emitter mesa separated by a SiO.sub.2 side wall of the emitter mesa. This may necessitate other masks to form the metal delineation from the emitter electrode and the base electrode, and may form steps of the delineation metals at the ends of the emitter mesa and the base mesa, which may incur breakage of the delineation metal. In this process, the base electrode is formed before the emitter electrode is formed. Hence, there may be limitation in the types of the base electrode metals due to the difference in alloying temperature of the base and emitter electrode.